make yer own
Hey man, there are pictures of them on the net but for whatever reason, that website is down and still down today. So I can't give you a picture but I think you can imagine what they should look like. I would just make them with a little smaller to cover the hole but not rub the paint. and of course, you need one for each side.
I would also use an o ring between the bolt head and plexiglass as a spacer along with a big washer to spread the pressure. This for vibration and pressure absorbtion as you don't want to overtighten it and fracture a vent.
This is where the majority of you air is going so blocking it holds it a little longer and forces more thru your knee vents. Just keep an eye on your temp gauge but this seems to have been done so many times someone is now manufacturing them.